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This article mainly explains "how to apply the filter, monitoring and interception technology of JavaEE". The content in the article is simple and clear, and it is easy to learn and understand. Please follow the editor's train of thought to study and learn "how to apply JavaEE filter, monitoring and interception technology".

1. Listener listener 1. Concept introduction

Three major components of JavaWeb: Servlet,Listener,Filter. A listener is a component that listens for changes in the state of related objects in an application.

2. Event source object

Refers to the object being monitored.

ServletContext

ServletContextListener lifecycle listening, which has two methods: call contextInitialized () at birth and contextDestroyed () at destruction

ServletContextAttributeListener property listener, which has three methods: add the attribute attributeAdded (), replace the attribute attributeReplaced (), and attributeRemoved () when the attribute is removed.

HttpSession

HttpSessionListener lifecycle listening: it has two methods, calling sessionCreated () when born and sessionDestroyed () when destroying.

HttpSessioniAttributeListener property listener: it has three methods: add property attributeAdded (), replace attribute attributeReplaced (), and remove attribute attributeRemoved ().

ServletRequest

ServletRequestListener lifecycle listening: it has two methods, calling requestInitialized () when born and requestDestroyed () when destroying.

ServletRequestAttributeListener property listener: it has three methods: add property attributeAdded (), replace attribute attributeReplaced (), and remove attribute attributeRemoved ().

3. Coding case

Related listener

TheContextListener

Public class TheContextListener implements ServletContextListener {@ Override public void contextInitialized (ServletContextEvent servletContextEvent) {System.out.println ("initialization: TheContextListener"); ServletContext servletContext = servletContextEvent.getServletContext (); servletContext.setAttribute ("author", "cicada");} @ Override public void contextDestroyed (ServletContextEvent servletContextEvent) {System.out.println ("destroy: TheContextListener");}}

TheRequestListener

Public class TheRequestListener implements ServletRequestListener {@ Override public void requestDestroyed (ServletRequestEvent servletRequestEvent) {System.out.println ("initialization: TheRequestListener");} @ Override public void requestInitialized (ServletRequestEvent servletRequestEvent) {System.out.println ("destroy: TheRequestListener");}}

TheSessionListener

Public class TheSessionListener implements HttpSessionListener {@ Override public void sessionCreated (HttpSessionEvent httpSessionEvent) {System.out.println ("initialization: TheSessionListener");} @ Override public void sessionDestroyed (HttpSessionEvent httpSessionEvent) {System.out.println ("destroy: TheSessionListener");}}

RequestAttributeListener

Public class RequestAttributeListener implements ServletRequestAttributeListener {@ Override public void attributeAdded (ServletRequestAttributeEvent evt) {System.out.println ("Request add attribute:" + evt.getName () + ";" + evt.getValue ());} @ Override public void attributeRemoved (ServletRequestAttributeEvent evt) {System.out.println ("Request remove attribute:" + evt.getName () + ";" + evt.getValue () } @ Override public void attributeReplaced (ServletRequestAttributeEvent evt) {System.out.println ("Request replacement attribute:" + evt.getName () + ";" + evt.getValue ());}}

Web.xml profile

Com.node05.servlet.listener.TheContextListener com.node05.servlet.listener.TheSessionListener com.node05.servlet.listener.TheRequestListener com.node05.servlet.listener.RequestAttributeListener 1

Test interface

Public class ListenerServletImpl extends HttpServlet {@ Override protected void doGet (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {response.setContentType ("text/html;charset=utf-8"); / / 1. Get TheContextListener initialization data ServletContext servletContext = this.getServletContext (); String author = String.valueOf (servletContext.getAttribute ("author")); System.out.println ("TheContextListener Author:" + author) / / 2. Set Request attribute to request.setAttribute ("mood", "smile"); request.setAttribute ("mood", "agitated"); / / 3. Create Session and expire in 1 minute. Call to destroy HttpSession session = request.getSession (true); session.setAttribute ("casually", "casually"); response.getWriter () .print ("Hello:Listener"). Filter filter 1. Brief introduction to filter

When the client requests a Servlet, the relevant Filter is executed first. If the Filter passes, the Servlet; that executes the request is inherited. If the Filter fails, the Servlet requested by the user will not be executed. Filters can intercept requests and responses dynamically.

2. Filter interface

The Filter interface defines three core methods.

Init ()

When the application starts, the server instantiates the Filter object, calls its init method, reads the web.xml configuration, and completes the initialization loading of the object.

DoFilter ()

For the actual filtering operation, when the request reaches the server, the Servlet container will first call the filter's doFilter method.

Destroy ()

The container calls this method before destroying the filter to release the resources consumed by the filter.

3. Coding case

Write a filter

Public class ThePrintLogFilter implements Filter {@ Override public void init (FilterConfig filterConfig) throws ServletException {String myName = filterConfig.getInitParameter ("myName"); System.out.println ("myName:" + myName);} @ Override public void doFilter (ServletRequest servletRequest, ServletResponse servletResponse, FilterChain chain) throws IOException, ServletException {HttpServletRequest request = (HttpServletRequest) servletRequest; HttpServletResponse response = (HttpServletResponse) servletResponse String name = request.getParameter ("name"); if (! name.equals ("cicada")) {response.getWriter (). Print ("User Error!"); return;} chain.doFilter (servletRequest,servletResponse);} @ Override public void destroy () {System.out.println ("ThePrintLogFilter destroy ()");}}

Web.xml profile

ThePrintLogFilter com.node05.servlet.filter.ThePrintLogFilter myName cicada thePrintLogFilter / filterServletImpl

Test interface

Public class FilterServletImpl extends HttpServlet {@ Override protected void doGet (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {response.setContentType ("text/html;charset=utf-8"); response.getWriter () .print ("Hello:Filter");}} III. Interceptor interceptor

The interceptor Interceptor in Spring framework is similar to the filter Filter in Servlet, which is mainly used to intercept user requests and deal with them accordingly. For example, the interceptor can verify permissions, log the request information, and determine whether the user is logged in or not. Request forwarding does not perform interception and filtering; redirection performs interception and filtering.

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